Peak detecting analogue to digital convertors (“ADCs”) are known and are described, for example, in U.S. Pat. No. 8,063,358 (Micromass). Peak detecting analogue to digital convertors have proven a useful device for enhancing the dynamic range, resolution and mass accuracy of orthogonal acceleration Time of Flight mass spectrometer instruments (“oa-ToF MS”).
Whilst these enhancements have resulted in improved measurements the approach is not without some drawbacks.
One drawback of the known approach (and all ADC based systems) is the loss of accurate intensity and time measurements when the vertical range of the analogue to digital convertor is exceeded i.e. when the analogue to digital convertor is suffering from saturation effects. This is a particular problem for Time of Flight analysers with asymmetric arrival time distributions (“ATDs”) and for ion detectors with asymmetric ion response profiles as the asymmetries result in time measurement shifts when the analogue signals exceed the vertical range of the analogue to digital convertor.
The approach described in U.S. Pat. No. 8,063,358 (Micromass) converts a detected ion peak into an intensity and arrival time value and results in improved performance relative to other height based approaches as the ion signals go into saturation and exceed the vertical range of the analogue to digital convertor. Whilst these improvements cause the system to fail in a more controlled manner it is still ultimately limited.
A second drawback which is specific to the approach described in U.S. Pat. No. 8,063,358 (Micromass) concerns the inability of the peak detection process to distinguish between multiple closely spaced (in time) ion response signals. In these situations, two or more closely spaced ion arrival events are interpreted as a single ion arrival event by the peak detection process and the two events are assigned a single arrival time and intensity value. This problem occurs more often when the ion response profiles are comparable with or greater than the analyser arrival time distribution (“ATD”).
WO 2010/136765 (Micromass) discloses a method of processing mass spectral data wherein mass spectral data is filtered out as noise if the area of an ion peak is determined to be less than a threshold peak area.
US 2014/005954 (Micromass) discloses a method of processing LC-ToF MS data in which a 2D dataset is produced, and 2D features are detected in the dataset to produce a list of regions of interest. For each region of interest, a corrected time of flight measurement and a corrected intensity are inferred which may involve suppression or rejection of detected peaks arising from interfering species and/or overlapping regions of interest.
It is desired to provide an improved method of mass spectrometry.